1. Field of the Invention
The present invention relates to a current limiter circuit in a multiphase DC/DC converter used as a power supply in a personal computer or the like.
2. Description of the Prior Art
As IT (information technology) equipment evolves in functionality, processing speed, and scale, it requires more and more current from power supply circuits. In particular in personal computers, with ever increasing clock speeds of CPUs and with diversification of peripheral devices that are supplied with power from personal computers, it is nowadays common that a load current of the order of a few tens of A to over 100 A is required.
Such personal computers are increasingly operated from built-in batteries rather than from commercially distributed power, and it is difficult for a single DC/DC converter to supply large current as mentioned above. Moreover, to eliminate ripples appearing in the supply voltage, a high-capacitance capacitor or a combination of many capacitors is required, which hampers miniaturization of equipment such as personal computers in which portability matters. Under these circumstances, multiphase DC/DC converters are used that are composed of a plurality of DC/DC converters connected in parallel with one another and operated with their output phases shifted relative to one another so as to produce large current with an improved ripple factor.
On the other hand, as increasingly large current is handled, from the viewpoints of protecting the circuit and securing satisfactory safety for the user, it is also important to detect and prevent as early as possible a short circuit or overcurrent resulting from an accident, fault, or inappropriate operation.
FIG. 4 is a block diagram of an example of a conventional power supply circuit provided with an overcurrent prevention circuit. Of many known types and configurations of circuits for detecting and preventing overcurrent, the power supply circuit shown in FIG. 4 adopts one in which an overcurrent detection circuit detects the voltage appearing across a resistor R in proportion to the current flowing through a load, and the detected voltage is fed to an unillustrated overcurrent protection circuit provided in a DC/DC converter to limit or cut off the current. FIG. 4 shows an example of overcurrent detection and overcurrent prevention in a single DC/DC converter. In a multiphase DC/DC converter provided with a plurality of DC/DC converters connected in parallel with one another, current is detected to prevent overcurrent for each of those DC/DC converters.
As described above, conventionally, even in a multiphase DC/DC converter, current is detected for each DC/DC converter. Thus, when electrical characteristics of individual converters vary due to variations in the characteristics of circuit elements and in temperature, and as a result the current limits set in the individual converters vary, for example, the sum of the load currents actually output from the individual converters may exceed the prescribed sum. This overloads the circuit elements provided on the primary side of the converters or the battery, eventually damaging them or shortening their operating lives. In addition, since current larger than the prescribed level flows through the load. This overloads the load, causing similar problems on this side, too.
On the other hand, when the current limit of one of the converters happens to fall below the prescribed current, overcurrent prevention may be invoked needlessly. This momentarily increases the other converters' share of the load current, adversely affecting the circuit elements provided on the secondary side of the converters which have to temporarily share the load, and thereafter invokes overcurrent protection in all of the converters, leading to an unnecessary shut-off of supply power.